The present invention is directed to downlink transmission power control and specifically to such power control related to Universal Terrestrial Radio Access (UTRA) Time Division Duplex (TDD) Downlink (DL) Power Control (PC).
Closed loop power control (also called inner loop control) is used for downlink (DL) transmission power control of the base station (BS). Such power control should minimize DL transmitted energy (minimize the signal-to-interference (SIR) at the user equipment (UE)) while maintaining quality of service at the UE. It should also maintain received signal power at the UE at a constant level at all times.
In operation the user equipment (UE) measures the signal-to-interference ratio (SIR) and according to that measurement, sends a transmission power control command (TPC) to the base station (BS). The base station then adjusts transmission power (TxP) according to the TPC command sent by the UE using a power control step size that is typically signaled from the radio network controller (RNC). The RNC ensures that the BS power level is within limits set by the network configuration. Thus there are certain power control limits, i.e. maximum and minimum transmission powers, that the BS cannot exceed. These power limits are set by radio resource management algorithms. It is of course desirable to adjust the base station transmission power as rapidly as possible in order to have the optimum transmission power level for the UE. However, if the signal-to-interference ratio (SIR) indicates a poorly received signal by the base station, then it may not be prudent to rapidly change the transmission power level, either up or down based upon the TPC command due to the fact that the TPC command may be incorrect. In fact, if the BS misreads the TPC command and interprets it as an UP command, and if the step size for increasing the base station transmission power is large, then a much higher base station transmission power than otherwise necessary for the UE can occur, resulting in degradation to other UEs interfacing with the BS. It is therefore desirable to be able to adjust the base station transmission power in a manner as commanded by the TPC command, but taking into account the reliability of that command based upon a measurement of the quality of the received signal.
The present invention describes a system and method for transmission power control step size selection which is based on received signal quality and which provides for the ability to independently adjust both the UP step size (StepU) and the DOWN step size (StepD) based upon the quality of the received signal at the base station. By so doing, the adjustment in the base station transmission power is able to minimize the UE receiver signal-to-interference ratio (SIR) of the downlink (DL) associated with a time division duplex (TDD) system. Furthermore, the present invention presents a rule for the behavior of the base station when the reliability of the transmission power control command is low.
In the present invention, when the base station (BS) receives a power control command (TPC) from a user equipment (UE), it evaluates the reliability of the transmitted command based upon the received signal quality of that transmission from the UE. Received signal quality can be based upon signal-to-interference ratio (SIR), or bit error rate (BER). Based on the received signal quality, the system determines the size of the UP step size (StepU) and the size of the DOWN step size (StepD) independently by comparing the received signal quality to different threshold values. In this way, maximum system capacity is realized. It has been found that under good signal conditions the UP and DOWN step sizes can be equal and can both be relatively large (such as 3 dB), but there are also situations where it has been found to be beneficial to choose unequal step sizes for UP and DOWN step sizes. Furthermore, it has been determined that although errors in the TPC command itself can impact the selection of the UP and DOWN step sizes, such errors nevertheless have not been found to change the determination that under certain circumstances the UP and DOWN step sizes should in fact be chosen independently.
Furthermore, under bad reception conditions, such as low SIR for the BS, it has been found beneficial to choose small step sizes for both the UP and DOWN step sizes. It has also been found that when an erroneous TPC command is received (such as the command having the value xe2x80x9c01xe2x80x9d or xe2x80x9c10xe2x80x9d), that it is beneficial to interpret such an erroneous command as an UP command (that is, for the base station to increase its downlink transmission power).
More particularly, in a Universal Terrestrial Radio Access (UTRA) time division duplex (TDD) system, users are separated both in the code domain and in the time domain. Due to the time division mode, channel conditions can fluctuate significantly and to such an extent that a slow power control speed (such as 100 Hz) is unable to correct for fast channel fading. This situation is unlike a Wideband Code Division Multiple Access (WCDMA) frequency division duplex (FDD) system where inner loop power control typically has a speed of 1500 Hz. Thus for time division duplex users, it is high desirable to be able to react to rapidly fluctuating conditions. In order to achieve this goal, adaptive power control step sizes for the base station downlink transmission power control are necessary.
When larger step sizes than 1 decibel (dB) are used, the system is more vulnerable to errors in transmission power control (TPC) commands. That is, if the UE is not sending a TPC command requesting an increase in downlink transmission power, but if the BS misinterprets the TPC as an UP command due to bad signal quality, and if for instance, a 3 dB step size is used for increasing the downlink transmission power; if a succession of these power control commands are received erroneously, it is possible that the base station could increase the downlink transmission power to this UE to such an extent that it adversely affects other UEs.
However, by controlling the downlink transmission power step sizes based upon received signal quality of the signal from the UE to the BS, system performance can be increased by using larger step sizes than 1 dB (not necessarily equal to each other) for UP and DOWN changes to the DL transmission power when it is determined that the received signal quality is good while using smaller step sizes (which may also be different in value for UP and DOWN TPC commands) when poorer received signal quality is determined to the BS.
In prior art techniques, the same step size is used for both UP and DOWN TPC commands, such that both the UP and DOWN step sizes could not be changed independently of each other. Thus in UTRA FDD which uses the same type of closed loop power control as used in UTRA TDD, asymmetrical step sizes are not needed since transmission is continuous and power control commands can be sent in every slot, thus providing that power control is better able to follow channel conditions even though using small step sizes for adjustment of the downlink transmission power. However as noted above, in UTRA TDD, the fast fading that can occur in the channel can only be countered in prior art devices by transmitting excessive power by the base station so as to compensate for the slow closed loop power control. This naturally leads to excessive inter-cell interference and is therefore highly disadvantageous.
The present invention provides the ability to modify the step size for both UP and DOWN downlink power control in response to current channel conditions perceived by the base station. When received signal quality is determined to be good, higher than 1 dB step sizes (StepU and StepD) can be used for both increasing downlink transmission power and for decreasing downlink transmission power. Furthermore, StepU and StepD need not be the same; that is, there may be asymmetrical steps for increasing or decreasing downlink transmission power.
However, during moderately bad or bad signal quality situations (channel conditions) asymmetry of step sizes can be eliminated and small step sizes for StepU and StepD can be used to minimize the effect of possible receipt of erroneous power control commands from the UE. This methodology is used only in the base stations and therefore does not require any additional signaling. Furthermore, this methodology does not require the radio network controller (RNC) to signal to the base station the step size to be used by the base station since the base station is able to select both the UP and DOWN step sizes independently of the RNC based upon perceived channel quality. Thus the need for additional signalling over the Iub interface (between the BS and RNC) is eliminated. However, the RNC continues to control the BS behavior to the extent that the maximum transmission power control step size is signalled from the RNC to the BS. According to current 3GPP specifications, the RNC signals the used step size to the BS, so that this value can be interpreted as a maximum step size when the BS has the ability (as in the present invention) to select appropriate step size independently based on received signal quality.
Although it might be, difficult to predict optimal step sizes for UP and DOWN power control in view of different environmental and channel conditions, it has been found through experimentation that there is a clear advantage in the potential use of asymmetrical step sizes. Nevertheless, selection of step size sizes for both UP and DOWN power control typically requires a channel dependent optimization.
Furthermore in the methodology according to the present invention, signal-to-interference ratio (SIR) measurement accuracy is not analyzed, but at reasonable operation levels the SIR or BER measurement is deemed reliable and thus the effect to system performance is small. In other words, at reasonable operation levels the UEs and BSs are configured so as to perform accurate measurements so that the effect to system capacity will be small, i.e. degradation in system performance should be very small because of any inaccuracy in various measurements.